The progression of prostate adenocarcinoma from androgen dependence to androgen independence is defined as clinical relapse from androgen ablation, the standard therapy for prostate cancer, and marks the incurable form of this disease. A large body of research supports a continued role for the androgen receptor (AR) in androgen independent disease although the mechanisms are poorly understood. One mechanism for AR action in androgen independent prostate cancer is via the upregulation or increased activity of AR coregulatory and modulatory proteins. We identified gene expression changes associated with progression to androgen independence by gene profiling analyses of the androgen dependent human prostate cancer cell line, LNCaP, and an androgen independent subline, LNCaP-R1, using high density microarrays. One of the most striking changes was an 8.5-fold up-regulation of vav3, a Rho GTPase guanine nucleotide exchange factor, in the androgen independent cells. As a putative protooncogene, vav3 is an intriguing candidate in prostate cancer. RNAse protection and immunoprecipitation assays confirmed that vav3 is elevated in androgen independent prostate cancer. Vav3 enhances AR transcriptional activity in several AR-expressing prostate cancer cell lines and in different promoter contexts. Further, a constitutively active vav3 mutant enhances growth of prostate cancer cells in vitro and a dominant negative transcript variant, vav3.1, inhibits growth. Our preliminary data prompted the following hypothesis, that vav3 up-regulation or increased activity promotes progression to androgen independence. We will address the following questions in this R21 application: 1. Are vav3 levels or activity induced in androgen independent prostate cancer in vivo? 2. Is vav3 activation necessary or sufficient for androgen independent tumor formation? We will use three different human prostate xenografts (LNCaP, LAPC4 and LAPC 9) that mimic clinical progression to androgen independence to define the role of vav3. The specific aims are: I. Examine vav3 levels and AR target gene expression during progression of prostate cancer xenografts LNCaP, LAPC4 and LAPC9 to androgen independence. II. Determine the role of vav3 signaling in progression to androgen independence by examination of androgen independent tumor formation by LNCaP cells expressing a constitutively active vav3 mutant and LNCaP-R1 cells expressing the dominant negative vav3.1